In the course of completing and maintaining subterranean wells, a number of operations are performed which require the introduction of fluids, generally termed completion fluids, into the well bore and into the producing formation. One of the common completion techniques performed on a well prior to placing it into production is gravel packing in which a slurry containing gravel is injected into the well to provide an in situ filtration medium to remove sand fines from produced fluids.
Subsequent to the gravel packing operation, a fluid such as water is introduced into the well to flush out excess gravel from the work string which is suspended within the bore of the production tubing. After the excess gravel slurry is flushed from the production tubing and the work string, the production tubing and the work string are filled with more dense completion fluids to prevent loss of produced fluids when the work string is withdrawn from the well prior to commencing production. After the well has been flushed with water and then filled with completion fluids as aforesaid, the work string, typically consisting of a service seal unit (cross-over tool), a sleeve valve shifter, and a wash pipe are withdrawn from the well bore leaving production packers, the closed sleeve valve and sand screens in place as functional parts of the production equipment.
Typically, the removed work string and its associated components contain large quantities of completion fluids which drain from the component tools into the annulus between the well casing and the work string as they are withdrawn from the well.
Because completion fluids are expensive and also possibly damaging to the producing formation, it is desirable to prevent the loss of completion fluids into the producing formation. It is also desirable to pressure test the tubing string to insure against the presence of leaks in the production tubing string prior to commencing production.
Typically, the processes of prevention of loss of completion fluids and of pressure testing the tubing is accomplished by the inclusion of an isolation valve with a frangible sealing member which is run in the hole as a part of the production tubing.
Conventional frangible isolation valves typically have frangible sealing elements made of glass or metal and are equipped either with an elastomeric hinge means which is bonded directly to said glass or metal sealing element such as in U.S. Pat. No. 4,813,481, or with retaining rings, springs or clips such as is disclosed in U.S. Pat. No. 4,216,830, into which said glass or metal sealing element is mounted said rings, springs or clips also functioning as hinge means. Such conventional frangible sealing elements are subject to abrasion, pitting and scarring which impairs sealing ability, and, after the sealing element has been broken, said hinge means remain down hole and frequently at least partially obstructs the well bore thereby increasing the difficulty with which subsequent down hole operations may be conducted.
In addition to the above difficulties which are associated with conventional frangible sealing elements, such valves also typically employ a resilient tapered sealing member in the valve seat such as that disclosed in U.S. Pat. No. 4,813,481. However, such resilient tapered valve seats in combination with the frangible sealing element often do not provide a reliable seal at the high pressures which are applied to the tubing during a pressure test thereof because the resilient sealing member tends to extrude into the bore of said valve seat.
Other U.S. patents which disclose isolation valves of the same general type as that disclosed in this Specification include U.S. Pat. Nos. 4,154,303; 4,160,484; 4,423,773; 4,433,702; 4,541,484; 4,597,445; 4,691,775.